AbstractTrichophyton ajelloi, a fungus isolated from soil, caused high larval mortality in Anopheles stephensi and Culex quinquefasciatus in the laboratory. The efficacy and toxicity of T. ajelloi against An. stephensi and Cx. quinquefasciatus were evaluated at 6 different concentrations. At different concentrations of T. ajelloi, chitin synthesis in larval cuticle was adversely affected. Larvae of 3rd-stage An. stephensi were more susceptible to T. ajelloi than were larvae of 1st, 2nd, and 4th stages as calculated by probit analysis. Larvae of Cx. quinquefasciatus were relatively resistant to infection by T. ajelloi.